subtraction of same element handled correctly

mankoff · mankoff · commit 46c2efd97634 · 2014-03-24T00:25:22.000-04:00
diff --git a/quantities/tests/test_uncertainty.py b/quantities/tests/test_uncertainty.py
@@ -60,3 +60,10 @@ def test_uncertainquantity_divide(self):
         self.assertQuantityEqual((a/2).uncertainty, [0.05, 0.1 ]*pq.m)
         self.assertQuantityEqual(1/a, [1., 0.5]/pq.m)
         self.assertQuantityEqual((1/a).uncertainty, [0.1, 0.05]/pq.m)
+
+    def test_uncertainquantity_subtract(self):
+        import numpy as np
+        a = UncertainQuantity(1, 'm', 1)
+        b = a.copy() # different object
+        self.assertQuantityEqual(a-a, UncertainQuantity(0, 'm', 0))
+        self.assertQuantityEqual(a-b, UncertainQuantity(0, 'm', np.sqrt(2)))
diff --git a/quantities/uncertainquantity.py b/quantities/uncertainquantity.py
@@ -102,7 +102,10 @@ def __radd__(self, other):
     @scale_other_units
     def __sub__(self, other):
         res = super(UncertainQuantity, self).__sub__(other)
-        u = (self.uncertainty**2+other.uncertainty**2)**0.5
+        if self is not other:
+            u = (self.uncertainty**2+other.uncertainty**2)**0.5
+        else:
+            u = self.uncertainty*0
         return UncertainQuantity(res, uncertainty=u, copy=False)
 
     @with_doc(Quantity.__rsub__, use_header=False)
